Methods for the characterization of very large segments of DNA have recently been used to determine the physical maps of the complete genomes of several organisms, including E. coli and the nematode C. elegans. Continued development of the methods of cosmid mapping, yeast artificial chromosome construction and pulsed field gel eletrophoresis strongly supports extension of such studies to large eukaryotic organisms, such as the human. This proposal describes the use of these approaches for the characterization of the human X chromosome, with special emphasis on the tip of the long arm spanning the region from the markers HPRT to G6PD and including the Fragile X locus. The proposal includes the combined use of cosmid fingerprinting and mapping together with YAC clone characterization and alignment to produce not only a physical map of the chromosome but also a contiguous set of reagents for the rapid isolation of markers from anywhere on the X chromosome.